The events of Sep. 11, 2001 and the explosion in “identity theft” have raised the need for reliable personal identification. There exists a need in society to positively identify, or at least, to authenticate a person's identity in the course carrying out certain transactions. These transactions range from purchasing items with a credit card to boarding commercial airplanes. This need is currently met superficially through looking at someone's driver's license, asking for a simple PIN code, and other similar but inadequate means. A more reliable means is to use a biometric identifier as one criterion to authorize the transaction.
Recently, a slew of biometric identification methods have been proposed, including iris scans, DNA matching, heart-beat patterns, and many other esoteric approaches. On the other hand, the human fingerprint has been an accepted method for identification and matching of persons for well over 100 years, and has withstood the test of time through numerous legal and court cases. In order for fingerprint identification to be widely used on hand-held devices, credit and debit cards, and identification badges and cards, the fingerprint sensor must be low cost, thin, mechanically conforming, mechanically flexible, and provide adequate resolution. Prior art fingerprint sensors fail to satisfy all of these requirements as evidenced by the lack of fingerprint sensors on commercially produced hand-held devices, credit and debit cards, and identification badges and cards.
There are several methods for capturing fingerprints: physical (inkpads), optical, electronic optical scan, capacitive and specialized materials. The inkpad method is the oldest method whereby the finger's pattern is transferred to a paper record using ink and finger pressure. The optical method uses a film or electronic camera to capture an optical image of the fingerprint. This is typically used in criminal investigations to document latent fingerprints from a crime scene. The electronic optical scan method involves the scanning of a fingerprint through its reflection of light, which is transferred directly from the subject finger to the opto-electronic sensor. This method has been in use in numerous government applications, including the issuance of drivers' licenses. These sensors must be flat and rigid. As a result, they are not suitable for use on most hand-held devices, credit and debit cards, and identification badges and cards.
The capacitive method is a more recent technique that is based on the measurement of the varying electric field produced by the fingerprint ridges. One example of this method is disclosed in U.S. Pat. No. 7,250,774. The sensor is a made using a semiconductor chip. The physical size of the sensor chip is typically at least as large as the area of the fingerprint to be captured. As a result, semiconductor-based sensors cannot be low cost because of the process complexities and the low number of sensors that can be made from a single silicon wafer. Hence, these devices cannot take advantage of the cost reduction curve due to integration that is characteristic of other semiconductor devices (human fingers are not getting smaller). Finally, these semiconductor sensors are made on a rigid substrate that requires a strong and flat surface for mounting in order to prevent the sensor from fracturing. Although most semiconductor sensors are inherently thin, the requirement for a rigid mounting substrate adds to the overall mounting platform thickness. These sensors must also be flat. As a result of these requirements and the cost of the sensor, they are not suitable for use on most hand-held devices, credit and debit cards, and identification badges and cards.
Various semiconductor-based sensors have been developed to use smaller sensor arrays (a small number of sensing lines in one dimension across the sensor) to capture images as the finger is swiped across the sensor that are assembled together create an image of the fingerprint (e.g., U.S. Pat. No. 6,580,816). The sensors require sophisticated processing software/hardware to assemble the individual images together into a complete image of a fingerprint. Although this design addresses the cost issue, it makes the mechanical reliability of this arrangement even worse than the more rigid two-dimensional array. These sensors must also be flat. As a result of these requirements and the cost of the sensor, they are not suitable for use on most hand-held devices, credit and debit cards, and identification badges and cards.
More recently, semiconductor based fingerprint sensors have been manufactured on flexible substrates, using amorphous silicon or other resistive material deposited on top of a variety of substrate materials (e.g., U.S. Pat. No. 6,680,485 and U.S. Published Patent Application No. 20060273417). Some of these devices are active sensors that are costly to manufacture and have high power requirements. Other devices use polymeric transistors that are unreliable in some environmental conditions. U.S. Pat. No. 6,680,485 attempts to address the flexibility and cost constraints required to put fingerprint sensors on hand-held devices, credit and debit cards, and identification badges and cards.
A number of other techniques have been proposed to manufacture electronic fingerprint sensors, although none of these are in widespread use commercially. Some of these techniques include the use of electro-luminescent materials (e.g., U.S. Pat. No. 7,248,298), piezo-electric materials (e.g., U.S. Pat. No. 5,515,738), magneto-resistive materials (e.g., U.S. Pat. No. 7,077,010), and others. These sensors are not suitable for use on hand-held devices, credit and debit cards, and identification badges and cards.
Unlike fingerprint sensors, low resolution pressure sensors have been developed using lower cost sensor elements (e.g., U.S. Pat. Nos. 5,033,291 and 6,964,205) and composite materials (e.g., U.S. Pat. Nos. 4,644,101 and 6,915,701 and 7,059,203 and 7,080,562 and 7,260,999). These sensors are typically used for pressure responsive input devices (e.g., U.S. Pat. No. 4,644,101), pressure distribution sensors (e.g., U.S. Pat. Nos. 5,033,291 and 6,964,205), and shear force sensors (e.g., U.S. Pat. No. 6,915,701).
U.S. Pat. No. 4,644,101 discloses a position sensor assembly which comprises a composite layer medium including electrically conductive magnetic particles in a nonconductive matrix material. The particles are aligned into chains extending across the thickness of the layer, and the chains include a non-conductive gap which is bridged upon application of sufficient pressure. The medium is sandwiched between sheet electrodes such that electrical measurements can be taken at specific points at the periphery of the sensor to determine where the pressure is being applied. Similarly, U.S. Pat. No. 6,915,701 discloses the use of non-random patterns of conductive particles to form columns having defined orientations in a non-conductive matrix. Although these sensors provide signals that are indicative of the position of a locally applied pressure, it cannot capture an image of a fingerprint.
U.S. Pat. No. 7,059,203 discloses a physical sensor having a pressure sensing layer and electrical insulating layers which are integrally formed on opposite two surfaces of the pressure sensing layer, respectively. The pressure sensing layer has a matrix comprising glass and electrically conductive particles dispersed in the matrix. One pair of electrodes are disposed on opposite sides of the pressure sensing layer such that the electrical resistance of the pressure sensing layer between the electrodes is changed by application of a stress on the electrical insulating layers. Although this sensor provides high precision as to the amount of pressure exerted on the sensor, it cannot capture an image of a fingerprint.
U.S. Pat. No. 7,080,562 discloses a pressure conduction sensor that includes a pair of locally resilient conductive layers and a locally resilient pressure conduction composite disposed between and contacting both conductive layers. The pressure conduction composite is composed of a plurality of conductive particles electrically isolated within a non-conductive matrix. The conductive particles are loaded so as to have a volume fraction approaching the critical percolation threshold of the material system and exhibit a conductance that greatly increases with pressure. Multiple sensors that are insulated from one another can be arranged to form one or more arrays including planar and conformal configurations. Such an array can be used for keypads, switches and intrusion detection systems, but cannot provide the resolution necessary to capture an image of a fingerprint.
U.S. Pat. No. 7,260,999 discloses a force sensing membrane that includes a pair of conductors and a composite material disposed between the conductors for electrically connecting the first and second conductors under application of sufficient pressure there between. The composite material contains conductive particles that have no relative orientation and are at least partially embedded in an elastomeric layer. The conductive particles are large enough in relation to the composite material such that substantially all conduction paths are through single particles, instead of many conductive particles. The sensor cannot capture an image of a fingerprint.
U.S. Pat. Nos. 4,856,993 and 5,033,291 and 6,964,205 disclose pressure sensors containing arrays of individual sensor elements. Each sensor element has a pressure sensitive resistive material disposed between and attached to both of the two electrodes. U.S. Pat. No. 4,856,993 discloses two sets of parallel electrodes which are each formed on a thin flexible supporting sheet. The electrodes are separated by a thin, pressure sensitive resistive coating and the sensor elements are created at the intersection points of the two sets of parallel electrodes. U.S. Pat. No. 5,033,291 also discloses two sets of parallel electrodes separated by a thin, pressure sensitive resistive coating on a thin flexible supporting sheet, but uses adhesive dots to separate the sensor elements from one another. The adhesive dots can be used to vary the sensitivity of the array by increasing/decreasing the number of adhesive dots in an area or by creating a small space between the pressure sensitive resistive materials attached to the two electrodes. The sensors in these two patents are created using a silk screening or printing process. U.S. Pat. No. 6,964,205 uses a complicated design of conductive traces and electrodes to form the sensor elements. All of these devices are designed to provide rather large sensor arrays to measure the pressure distribution between two opposing objects. The stated resolution between contact points for these processes is 0.050 inches or less. Although printing techniques have improved to provide better sensor element density since these patents issued, these processes are likely too expensive at the required resolution for use on hand-held devices, credit and debit cards, and identification badges and cards.
These previous designs do not have all the characteristics necessary for the fingerprint biometric to be employed ubiquitously in society (i.e., low cost, thin, mechanically conforming, mechanically flexible, adequate resolution). Accordingly, there is a need for a fingerprint sensor that is low cost, thin, mechanically conforming, mechanically flexible and provides adequate resolution such that it can be used on hand-held devices, credit and debit cards, identification badges and cards, and the like.